ATBECHDEDKESFRGBGRITLILUNLSEMCPTIESILTLVFIROMKCYALTRBGCZEEHUPLSKBAHRIS..MTNORSMESMMAKHTNMD..........
J
BDM Ver 0.1.67 (18 Oct 2017) - 1100000/0
3579337
EUROPEAN PATENT APPLICATION
A1
20191211
EP
18176550.4
20180607
en
en
en
20191211
201950
20191211
201950
H01Q 1/28 20060101AFI20181126BHEP
H01Q 1/52 20060101ALI20181126BHEP
H01Q 3/24 20060101ALI20181126BHEP
H01Q 3/26 20060101ALI20181126BHEP
H01Q 21/20 20060101ALI20181126BHEP
G01S 13/34 20060101ALI20181126BHEP
G01S 13/89 20060101ALI20181126BHEP
H01Q 23/00 20060101ALI20181126BHEP
H01Q 19/10 20060101ALI20181126BHEP
H01Q 19/17 20060101ALI20181126BHEP
G01S 7/40 20060101ALN20181126BHEP
de
RADARANTENNE ZUR VERWENDUNG IN EINEM MINIATURISIERTEN UNBEMANNTEN LUFTFAHRZEUG
en
RADAR ANTENNA FOR USE IN A MINIATURE UNMANNED AERIAL VEHICLE
fr
ANTENNE RADAR DESTINÉE À ÊTRE UTILISÉE DANS UN VÉHICULE AÉRIEN SANS PILOTE MINIATURE
NONE
Nederlandse Organisatie voor toegepast- natuurwetenschappelijk onderzoek TNO
101513296
P119330EP00
Anna van Buerenplein 1
2595 DA 's-Gravenhage
NL
Otten, Maternus Petrus Gerardus
c/o TNO / IP & Contracting, Anna van Buerenplein 1,
2595 DA 's-Gravenhage
NL
Maas, Arnoldus Petrus Maria
c/o TNO / IP & Contracting, Anna van Buerenplein 1,
2595 DA 's-Gravenhage
NL
Bolt, Roland Johannes
c/o TNO / IP & Contracting, Anna van Buerenplein 1,
2595 DA 's-Gravenhage
NL
V.O.
101564910
P.O. Box 87930 Carnegieplein 5
2508 DH Den Haag
NL
AL
AT
BE
BG
CH
CY
CZ
DE
DK
EE
ES
FI
FR
GB
GR
HR
HU
IE
IS
IT
LI
LT
LU
LV
MC
MK
MT
NL
NO
PL
PT
RO
RS
SE
SI
SK
SM
TR
BA
ME
KH
MA
MD
TN
The radar antenna contains a sandwich of two printed circuit boards between three conductive plates. A first printed circuit board comprises a preferably circular array of first antenna elements such as dipoles. A second printed circuit board, parallel to the first printed circuit board, comprises an array of second antenna elements. One of the array of first antenna elements and the array of second antenna elements is an array of transmission antenna elements and the other an array of reception antenna elements. The first printed circuit board is located below the second printed circuit board. Three conductive plates are used to shape the antenna patterns from the antenna elements so that the main lobes of the antenna patterns are directed obliquely downwards and the antenna patterns from the different array at least partly overlap, suppressing vertical side lobes. A first conductive plate separates the first and second printed circuit boards. A second conductive plate is located above the second printed circuit board, extending radially outward beyond the first conductive plate. A third conductive plate is located below the first printed circuit board. The first conductive plate extends radially outward beyond the third conductive plate.

Field of the invention
The invention relates to a radar antenna and a radar system using such an antenna that are suitable to be carried by a miniature UAV (Unmanned Aerial Vehicle).

Background art
Radar antennas carried as part of miniature UAVs, which typically weigh less than ten kilograms, and often less than one or two kilograms, are constrained by weight and size conditions. Furthermore, because of the partly unpredictable motion of such miniature UAVs, such antennas should preferably be able to support radar operation under rapidly varying orientations.
For larger aircraft, synthetic aperture radar (SAR) solutions are known. Typically, these involve an antenna that comprises a linear array of antenna elements, or a two dimensional array of antenna elements.
WO2011112084 describes a radar antenna with a plurality of reception and transmission antenna elements on an outward facing cylindrical surface, with transmission antenna elements spaced at fixed angle distances and groups of reception antenna elements in the spacings between the transmission antenna elements. This radar antenna makes it possible to receive radar reflections from any radial direction around the cylinder without requiring mechanical rotation. However the antenna is less suited for a surface mapping radar.

Summary
Among others, it is an object to provide for a compact radar antenna that is suitable for use on a miniature UAV and to provide a radar system comprising such a radar antenna.
A radar antenna according to claim 1 and a miniature UAV with such a radar antenna are provided. Herein a sandwich with three electrically conductive rings or plates are provided to direct the antenna pattern from antenna elements of a transmitter array and a receiver array on printed circuit boards between the rings or plates obliquely downward in an overlapping manner. The resulting radar antenna can be realized in a light weight and compact way, making the antenna suitable for use hanging from a miniature UAV for ground surveillance.
In an embodiment the lowest electrically conductive ring or plate has the form of a frustrated cone, widening in a direction away from the lowest printed circuit board. This makes it easier to obtain the desired antenna patterns.
The antenna elements in each of the arrays are preferably located at different angles from the central axis (i.e. so that lines from the different antenna elements to where the central axis intersects the plane of the printed circuit board form different angles with each other). This makes it possible to resolve reflections from different direction and to compensate for rotation of the UAV.
In an embodiment the array of first antenna elements and the array of second antenna elements are both circular arrays, the positions of the first antenna elements lying at the same first distance from the central axis, the positions of the second antenna elements lying at the same second distance from the central axis, at least the first and third electrically conductive ring or plate both having a circular outer perimeter. Thus, highly similar patterns in different directions can be realized. In a further embodiment, the second electrically conductive ring or plate may also have a circular outer perimeter. Preferably, the positions of the first antenna elements and/or the second antenna elements are located at evenly spaced angles from the central axis. This provides for an evenly distributed resolution.
In an embodiment the first antenna elements and/or the second antenna elements are dipole antenna elements. Thus each antenna element can be realized as a simple conductor structure on the printed circuit boards.
In an embodiment the radar antenna comprises an electrically conductive cylinder, concentric with the central axis, extending between the first electrically conductive ring or plate and third electrically conductive ring or plate, with a diameter smaller than the first distance or the lower limit of the first range of distances. This provides for an electromagnetic shield between the antenna elements on opposite sides of the array, and between optional transmission or reception electronics within the cylinder. Similarly an electrically conductive cylinder may be provided extending between the first electrically conductive ring or plate and the second electrically conductive ring or plate, with similar advantages.
In an embodiment the array of second antenna elements comprises more antenna elements than the array of first antenna elements. Because the array of second antenna elements can have a larger diameter than the array of first antenna elements space for more antenna elements is available. By using more antenna elements, more angular resolution is made possible. In a further embodiment, the larger number of second antenna elements may be used for reception antenna elements. Reflection signals from these antenna elements may be received in parallel and processed to realize higher resolution.
Preferably the antenna element within the first array and within the second array are all used for transmission or all used for reception, the different arrays being used for transmission and reception.
In an embodiment the radar antenna is part of a radar system with a processing system configured to synthesize a radar image of a surface below the radar system from reflected signals received by the reception electronics coupled to the antenna elements from one of the arrays in response to transmissions from the antenna elements of the other one of the arrays.
In an embodiment the radar system comprises an orientation sensor, the transmission electronics and/or the reception electronics and/or the processing system being configured to compensate radar operation for rotation sensed by the orientation sensor.

Brief description of the drawing
These and other objects and advantageous aspects will become apparent from a description of exemplary embodiments, with reference to the following figures
Figure 1a
shows a miniature UAV comprising an antenna
Figure 1b
shows an antenna structure in side view
Figure 2
shows an antenna structure in bottom view
Figure 3
shows an embodiment with interior cylinders
Figure 4
shows a radar system

Detailed description of exemplary embodiments
Figure 1a shows a miniature UAV comprising propellers 50, a UAV body 52 and an antenna structure hanging from UAV body 52. The antenna structure is configured to provide transmission and reception antenna patterns with main lobe directions 54, 56 that are directed obliquely downwards, for example at an angle to the vertical between forty five and seventy five degrees. Preferably, downward side lobes along the vertical direction should be minimal, to avoid reception of strong perpendicular reflection from the ground.
Figure 1b and 2 show an exemplary embodiment of an antenna structure in side view cross-section and bottom view respectively. The antenna structure comprises first and second, parallel printed circuit boards 10, 12 and first and second circular arrays of antenna elements on printed circuit boards 10, 12 respectively. In normal use, the surfaces of first and second printed circuit boards 10, 12 will be oriented horizontally, with second printed circuit board 12 above first printed circuit board 10, as shown in figure 1a. Preferably, all antenna elements in the first circular array have the same shape, e.g. a dipole shape, and similarly all antenna elements in the second circular array have the same shape. An arbitrary point on an antenna element may be referred to as the position of the antenna element, corresponding points on different antenna elements being referred to as the positions of the different antenna elements. The shapes of the antenna elements in the first circular array may be the same as those in the second circular array, but this is not necessary. Antenna elements 100, 120 will typically consist of conductor planes and/or tracks, e.g. dipoles, on the surface of first and second printed circuit boards 10, 12, but for the sake of illustration they are symbolically indicated as rectangles.
As shown in figure 2, a first circular array of transmission antenna elements 100 (only one labeled) along a first circle is provided on first printed circuit board 10. A second circular array of reception antenna elements 120 ((not shown in figure 2) is provided along a second circle on second printed circuit board 12. The first and second printed circuit boards 10, 12 may be circular as well. Preferably the positions of the antenna elements 100, 120 are evenly spaced along the circles. If the array contains N antenna elements, the positions of the antenna elements may be spaced by three hundred sixty degrees divided by N along the circle. In an embodiment, the first and second circular array contain sixteen and thirty two antenna elements respectively. Preferably, radar wavelengths are used that are larger than the spacing between the positions of the antenna elements, for example with a wavelength between the spacing and twice the spacing.
The first and second arrays are coaxial around a central axis perpendicular to the surfaces of printed circuit boards 10, 12. The radius of the first circular array, i.e. the distance from the central axis to the positions of transmission antenna elements 100 of the first circular array, is smaller than the corresponding radius of the second circular array, containing the reception antenna elements 120. When the first and second printed circuit boards are circular, their radii may be larger than those of the circular arrays.
Furthermore the antenna structure comprises a first and second circular electrically conductive plate 14, 16, in parallel with first and second printed circuit boards 10, 12. First and second circular electrically conductive plate 14, 16 may be made of metal. First electrically conductive plate 14 is located between first and second printed circuit boards 10, 12 and second printed circuit board 12 is located between first and second electrically conductive plate 14, 16, i.e. second electrically conductive plate 16 is located above second printed circuit board 12 in normal use. The antenna structure comprises a further electrically conductive plate 18 (not shown in figure 2), e.g. of metal, below first printed circuit board 10. Further electrically conductive plate will also be referred to as a beam shaper. Optionally, as shown further electrically conductive plate 18 has the form of a frustrated electrically conductive cone surface, first printed circuit board 12 being located between first electrically conductive plate 14 and further electrically conductive plate 18. The radius of the electrically conductive cone surface widens with increasing distance from first printed circuit board 12.
The antenna elements 100, 120 of one of the first and second array of antenna elements serve as transmission antennas and the antenna elements 100, 120 of the other array serves as array of antenna elements serves as an array of reception antennas. The elevation angle of the peak strength of the main lobes of the transmission and reception antenna patterns (the angle with the direction normal to the surface of the first and second printed circuit boards) is directed obliquely downward (e.g. at an angle between forty five and seventy five degrees with the vertical), and the main antenna pattern lobes of the reception and transmission antenna patterns at least partly overlap as a function of the elevation angle. The lobe width need not be very narrow. For example a lobe width of about forty degrees may be used. Preferably, downward transmission and/or reception along the vertical direction should be minimal, to avoid reception of strong perpendicular reflection from the ground.
It has been found that conductive plates 14, 16, 18 can be used to ensure these requirements. In general terms, elevation angle of the peak strength of the main lobe of the antenna pattern of antenna elements between an upper and lower conductive plate (e.g. plates 14, 16) becomes more downward directed when the radius of the upper conductive plate (e.g. plate 16) increases and more upward directed when the radius of the lower conductive plate (e.g. plate 14) increases. The effect of changes of the radius of a plate is strongest when the radius of the plate is about the same as the radius of the array of antenna elements between the plates. This allows tuning of the elevation angle of the peak strength of the main lobe. For much larger and much smaller radii the effect levels off. Typically, achieving an obliquely downward directed elevation angle of the peak strength of the main lobe, the radius of the upper conductive plate (e.g. plate 16) will be larger than the radius of the array of antenna elements and the radius of the lower conductive plate (e.g. plate 14) will be smaller than the radius of the array of antenna elements.
To reduce cross-talk between transmission and reception, the radius of first electrically conductive plate 14 should preferably be at least so large that first electrically conductive plate 14 blocks the direct line of sight between antenna elements on first printed circuit board 10 and second printed circuit board 12. Simulation and measurement have shown that first electrically conductive plate 14 does not need to extend further to achieve acceptable cross-talk reduction. Also, the design freedom of the radius of first electrically conductive plate 14 is limited because it acts as upper plate and as lower plate for the antenna elements on first printed circuit board 10 and second printed circuit board 12 respectively, with opposite effect on the lobe direction.
Using a further electrically conductive plate 18 in the form of a frustrated electrically conductive cone surface below first printed circuit board 10 has the advantage that it provides for an additional design parameter (the angle of the cone) that can be used to adjust the elevation angle of the peak strength of the main lobe to make the main antenna pattern lobes of the reception and transmission antenna patterns at least partly overlap as a function of the elevation angle and reduce vertical side lobes. Adjustment of the distance between first and second electrically conductive plates 14 and 16 and between first electrically conductive plate 14 and the top of further electrically conductive plate 18 may be used to adjust the impedance of the antenna elements. If the electrically conductive plates are too close to each other, the impedance may become very small. First and second printed circuit boards 10, 12 may lie midway between the adjacent conductive plates, or their position may be varied to adjust the impedance. Impedance adjustment may be used for example to match the antenna element impedance to the impedance of the transmitter and the receiver circuit. But the antenna pattern also depends on the distance between the plate. Therefore the radii of the plates may be selected for a pre-selected distance.
In the illustrated embodiment, the radius of first circular electrically conductive plate 14 is larger than the radius of the first circular array. The radius of second circular electrically conductive plate 16 is larger than the radius of the second circular array.
The actual dimensions of a concrete antenna may be selected experimentally or by means of simulation, by adjusting the dimensions to obtain at least partly overlapping main lobes in a desired elevation range, and optionally using the dimensions to optimize factors like lobe overlap, lobe width and suppression of lobes in undesired directions (e.g. directly vertical from the antenna). In an embodiment for a 9.8 GHz radar, the requirements were realized using radii of first and second electrically conductive plates 14 and 16 of seventy nine and a hundred twenty two millimeter and further electrically conductive plate 18 having a top radius of forty five millimeter and a bottom diameter of seventy millimeter.
An elevation angle of the peak strength of the main lobe of about fifty five degrees to the downward vertical was realized in this way. In this embodiment, the distance between first and second electrically conductive plates 14 and 16 and between first electrically conductive plate 14 and the top of further electrically conductive plate 18 is twenty millimeter, first and second printed circuit boards 10, 12 lying midway. It should be noted this combination of dimensions is merely an example. Different combinations of dimensions may also be used to obtain obliquely downward directed, at least partly overlapping antenna lobes. For a different frequency radar, the dimensions may be scaled accordingly.
A conical dielectric cover (not shown) may be used over printed circuit boards 10, 12 and first and second circular electrically conductive plate 14, 16 and optionally beam shaper 18. In this embodiment the perimeters of printed circuit boards 10, 12 and first and second circular electrically conductive plate 14, 16 preferably substantially lies on the cone.
In the illustrated embodiment first and second circular electrically conductive plate 14, 16 are flat plates and further electrically conductive plate 18 is cone shaped. Instead of a cone shape a curved shape may be used. Another alternative for the cone shape is a flat further electrically conductive plate 18 may be used, but this makes the design possibilities for beam shaping less flexible. Similarly, electrically conductive plate 14, 16 may deviate from flat shapes, e.g. by using a plate with a cone shaped or curved perimeter e.g. a downward or upward curved or angled perimeter. This may make beam shaping slightly more flexible. Figure 3 shows an embodiment wherein first and second electrically conductive cylinders 30, 32 have been added (e.g. metal cylinders). First and second electrically conductive cylinder 30, 32 extend between further electrically conductive plate 18 and first electrically conductive plate 14 and from first electrically conductive plate 14 to second electrically conductive plate 16 respectively. Second electrically conductive cylinder 32 electrically connects first and second electrically conductive plates 14, 16. First electrically conductive cylinder 30 electrically connects first electrically conductive plate 14 and further electrically conductive plate 18. In an embodiment, first and second electrically conductive plates 14, 16 and further electrically conductive plate 18 extend through the interior of first and second electrically conductive cylinder 30, 32. Alternatively, one or more of first and second electrically conductive plates 14, 16 and further electrically conductive plate 18 may be present only outside first and/or second electrically conductive cylinder 30, 32.
The axis of first and second electrically conductive cylinder 30, 32 coincides with the central axis of the circular arrays. The radius of first electrically conductive cylinder 30 is smaller than the radius of the first circular array, so that the transmission antenna elements 100 lie outside first electrically conductive cylinder 30. The radius of second electrically conductive cylinder 32 is smaller than the radius of the second circular array, so that the reception antenna elements 120 lie outside second electrically conductive cylinder 32. First and second electrically conductive cylinder 30, 32 serve as electromagnetic shields between antenna elements 100, 120 and electronics within first and second electrically conductive cylinder 30, 32, and counteract wave propagation between the conductive plates, e.g. from the side in the direction from which a reflected wave arrives to an antenna element on the opposite side of the plate. Preferably, the radius of second electrically conductive cylinder 32 is larger than the radius of first electrically conductive cylinder 30.
First and second printed circuit boards 10, 12 are mechanically connected to first and second electrically conductive cylinder 30, 32 respectively. In an embodiment, first electrically conductive cylinder 30 may comprise two cylinder parts, connected to the upper and lower surfaces of first printed circuit board 10. In another embodiment, first printed circuit board 10 may contain two or more parts fitted through slots in first electrically conductive cylinder 30. Mutatis mutandis, the same constructions may be used for second electrically conductive cylinder 32 and second printed circuit board 10.
Figure 4 shows a radar system, comprising the first and second circular array 46, 48, a processing system 40 and reception and transmission electronics 42, 44. Processing system 40 is coupled to reception and transmission electronics 42, 44. Although connection lines are shown, it should be noted that wireless coupling may be used and that processing system 40 may be located remotely from reception and transmission electronics 46, 48, e.g. in a ground based control unit. Processing system 40 may comprise one or more programmable computers and storage device(s) with computer programs to make the one or more programmable computers perform the functions of processing system 40.
Reception electronics 42 has inputs coupled to the reception antenna elements of second circular array 48. By way of example amplifiers have been shown between reception electronics 42 and the reception antenna elements, which may be considered to be part of reception electronics 42. Transmission electronics 44 has outputs coupled to the transmission antenna elements of first circular array 46. Transmission electronics 44 may contain output amplifiers (not shown) with outputs coupled to the transmission antenna elements.
Transmission electronics 44 may be located on first printed circuit board 10 at least partly inside first electrically conductive cylinder 30. Reception electronics 42 may be located on second printed circuit board 12 at least partly inside second electrically conductive cylinder 32. Processing system 40 may be located on either, or distributed over both or remotely.
In operation, a UAV carries the antenna structure. As shown in figure 1,the UAV may comprise a plurality of propellers attached to a body that carries the antenna structure, the plurality of propellers being configured to provide upward trust. Processing system 40 causes transmission electronics 44 to supply transmission signals to the transmission antenna elements in first circular array. In an embodiment transmission electronics 44 may be configured to generate frequency modulated transmission signals for FMCW (frequency modulated continuous wave) radar operation. In an embodiment transmission electronics 44 are configured to supply a transmission signal to a one of transmission antenna element at a time, successively to different ones of the transmission antenna elements to steer the transmitted beam in successively different directions. In another embodiment, the same transmission signal may be supplied simultaneously to a plurality of transmission antenna elements to form an antenna beam from plurality of transmission antenna elements, optionally with adapted phase and/or amplitudes to direct or shape the beam. The resulting beam is directed radially and downward from the antenna structure, with a peak as function of radial direction. Transmission electronics 44 may be configured to scan the beam direction along a series of radial directions.
Because the transmission antenna elements are located in an array at different angles from the central axis (i.e. so that lines from the different antenna elements to where the central axis intersects the plane of the printed circuit board form different angles with each other), the radial direction of the antenna pattern can easily be steered to compensate for rotation around the axial direction due to rotation of the UAV that carries the antenna structure. In an embodiment, the antenna structure contains a sensor for measuring a rotation angle of the antenna structure around the axial direction, the sensor being coupled to transmission electronics 44. The transmission electronics 44 may be configured to adapt the beam direction in response to sensed angle changes so as to compensate the radial direction the beam for rotation of the antenna structure.
Reception electronics 42 receives reflection signals from the reception antenna elements. Preferably, reception electronics 42 processes reflection signals from a plurality of reception antenna elements in parallel. When FMCW is used, the reflection signals may be mixed with a copy of the transmitted signals to obtain beat signals with frequency components corresponding to objects at different distances. By computing linear combinations of the reflection signals before or after mixing (preferably digitally), radial direction selective reflection signals may be computed. Because the reception antenna elements are located in an array at different angles from the central axis, signals from a combination of reception antenna elements can be used to resolve reflections from different radial directions. Reception electronics 42 or processing system 40 may be configured to compensate the reflection signals for sensed rotation, like the transmission signals, e.g. by summing reflection signals that have been multiplied by factors determined using the sensed rotation angle, according to predetermined functions of the rotation angle, or by accounting for the rotation in radar image computation. In addition, the UAV may be moved and processing system 40 may be configured to combine results from reflection signals obtained at different positions to construct a synthetic radar image.
It is preferred to use the antenna elements on first printed circuit board 10 as transmission antenna elements and the antenna elements on second printed circuit board 12 as reception antenna elements. Because second printed circuit board 12 has a larger radius, more antenna elements can be provided on second printed circuit board 12, enabling use of signal processing to obtain a higher resolution. Alternatively, the antenna elements on first printed circuit board 10 may be used as reception antenna elements and the antenna elements on second printed circuit board 12 as transmission antenna elements.
Although embodiments with circular arrays on printed circuit boards 10, 12 have been shown, wherein the distance from the position of each antenna element to the central axis of the array is the same, alternatively the distances may vary within a predetermined range of distances, such as in a polygonal or elliptical array, and/or the angular spacing between the positions of successive antenna elements may vary. It is preferred that the antenna elements are positioned so that, for any given orientation of the antenna structure the antenna elements in the arrays can be selected to transmit and receive waves in all radial directions relative to the antenna. The distances and/or spacing may be different on the first and/or second printed circuit board 10, 12.
1. A radar antenna, comprising
- a first printed circuit board, comprising an array of first antenna elements on the first printed circuit board, the positions of the first antenna elements lying at a same first distance, or all within a first predetermined range of distances, from a central axis that is perpendicular to the first printed circuit board, the first antenna elements lying in different directions from the central axis;
- a second printed circuit board, parallel to the first printed circuit board, the second printed circuit board comprising an array of second antenna elements on the second printed circuit board, the positions of the second antenna elements lying at a same second distance, or all within a second predetermined range of distances, from the central axis, in different directions from the central axis, the first distance or distances in the first range being smaller than the second distance or distances in the second range;
- wherein one of the array of first antenna elements and the array of second antenna elements is an array of transmission antenna elements and the other an array of reception antenna elements;
- a first electrically conductive ring or plate located between the first and second printed circuit boards, blocking lines of sight from the antenna elements of the first array to the antenna elements of the second array to,
- a second electrically conductive ring or plate, the second printed circuit board lying between the second electrically conductive ring or plate and the first electrically conductive ring or plate, the second electrically conductive ring or plate extending radially outward beyond the first electrically conductive ring or plate;
- a third electrically conductive ring or plate, the first printed circuit board lying between the third electrically conductive ring or plate and the first electrically conductive ring or plate, the first electrically conductive ring or plate extending radially outward beyond the third electrically conductive ring or plate.
2. A radar antenna according to claim 1, wherein the third electrically conductive ring or plate has the form of a frustrated cone, widening in a direction away from the first printed circuit board.
3. A radar antenna according to any of the preceding claims, wherein the array of first antenna elements and the array of second antenna elements are both circular arrays, the positions of the first antenna elements lying at the same first distance from the central axis, the positions of the second antenna elements lying at the same second distance from the central axis, at least the first and third electrically conductive ring or plate both having a circular outer perimeter.
4. A radar antenna according to any of the preceding claims, wherein the positions of the first antenna elements and/or the second antenna elements are located at evenly spaced angles from the central axis.
5. A radar antenna according to any of the preceding claims, wherein the first antenna elements and/or the second antenna elements are dipole antenna elements.
6. A radar antenna according to any of the preceding claims, comprising an electrically conductive cylinder, concentric with the central axis, extending between the first electrically conductive ring or plate and third electrically conductive ring or plate, with a diameter smaller than the first distance or the lower limit of the first range of distances.
7. A radar antenna according to claim 6, comprising transmission electronics on the first printed circuit board within the electrically conductive cylinder, the first antenna elements being transmission antenna elements coupled to the transmission electronics.
8. A radar antenna according to any of the preceding claims, comprising an electrically conductive cylinder, concentric with the central axis, extending between the first electrically conductive ring or plate and the second electrically conductive ring or plate, with a diameter smaller than the second distance or the lower limit of the second range of distances.
9. A radar antenna according to claim 8, comprising reception electronics on the second printed circuit board within the electrically conductive cylinder, the second antenna elements being reception antenna elements coupled to the reception electronics.
10. A radar antenna according to any of the preceding claims, wherein the array of second antenna elements comprises more antenna elements than the array of first antenna elements.
11. A radar system containing a radar antenna according to any one of the claims 1-10, transmission and reception electronics, the reception and transmission electronics being coupled to the arrays of first and second antenna elements respectively, or to the arrays of second and first antenna elements respectively.
12. A radar system containing a radar antenna according to claim 10, reception and transmission electronics and a processing system coupled to the reception and transmission electronics and configured to cause the transmission electronics to supply a transmission signal or signals to selectable ones or combinations of the array of first antenna elements, the reception electronics being coupled to the array of second antenna elements.
13. A radar system according to claim 11 or 12, comprising a processing system configured to synthesize a radar image of a surface below the radar system from reflected signals received by the reception electronics.
14. A radar system according to any one of claims 11-13, comprising an orientation sensor, the transmission electronics and/or the reception electronics and/or the processing system being configured to compensate radar operation for rotation sensed by the orientation sensor.
15. A miniature UAV comprising a plurality of propellers configured to provide upward trust, a UAV body connected to the propellers configured and the radar antenna of any one of claims 1-10, or the radar system of any one of claims 11-14, hanging from the UAV body with the first printed circuit board below the second printed circuit board.
P119330EP00
EP
18176550.4
Nederlandse Organisatie voor toegepast-natuurwetenschappelijk onderzoek TNO
DH
20181130
H01Q
G01S
US
2013106667
A1
FENN ALAN J [US] ET AL
20130502
Y
1-15
* paragraphs [0043] - [0048], [0052] - [0055]; figures 2,4,5,9,15,16,17 *
US
6104346
A
RUDISH RONALD M [US] ET AL
20000815
Y
1-15
* column 8, lines 13-37, 49-61; figures 8, 9, 14A, 14B, 16A,16B,18 *
* column 12, lines 34-47 *
* column 13, lines 4-19, 63-67 *
* column 14, lines 56-67 *
US
9640858
B1
ISLAM MD [US] ET AL
20170502
Y
6-9
* column 4, lines 52-63; figures 10,11,13,14 *
* column 8, lines 5-36 *
* column 9, lines 20-53 *
M.P.G OTTEN ET AL
Multichannel imaging with the AMBER FMCW SAR
10TH EUROPEAN CONFERENCE ON SYNTHETIC APERTURE RADAR
20140603
978-3-8007-3607-2
209
212
XP055522488
Y
11-15
* Section 1, Introduction;page 209, left-hand column, lines 1-9; figures 1,2, 7,8 *
* Section 2, The Amber System;page 209, left-hand column, lines 1-16 *
* Section 2, The Amber System;page 209, right-hand column, lines 1-4 *
* Section 4, New SAR modes with AMBER;page 211, lines 5-7 *
* Section 5, Real Time Processing;page 212, lines 4-10 *
US
4305078
A
JONES JR HOWARD S ET AL
19811208
A
1-15
* column 3, lines 3-22, 54--67; figure 5 *
Georgiadis, A
The Hague
20181123
US
2013106667
A1
20130502
NONE
US
6104346
A
20000815
US
6104346
A
20000815
WO
0028344
A1
20000518
US
9640858
B1
20170502
NONE
US
4305078
A
19811208
NONE

REFERENCES CITED IN THE DESCRIPTION
This list of references cited by the applicant is for the reader's convenience only. It does not form part of the European patent document. Even though great care has been taken in compiling the references, errors or omissions cannot be excluded and the EPO disclaims all liability in this regard.

Patent documents cited in the description
- WO
2011112084
A
[0004]